U.S. patent number 5,534,289 [Application Number 08/367,039] was granted by the patent office on 1996-07-09 for structural crack monitoring technique.
This patent grant is currently assigned to Competitive Technologies Inc.. Invention is credited to Wayne H. Bilder, Richard D. Granata, Henry Leidheiser, Jr..
United States Patent |
5,534,289 |
Bilder , et al. |
July 9, 1996 |
Structural crack monitoring technique
Abstract
A method is disclosed for monitoring a structure for the
formation of cracks and for providing protection of the structure
from the environment, including the following steps: applying to
the structure a first coating comprising microcapsules of a first
color, applying over the first coating, a second coating having a
second color, and identifying cracked portions which form in the
structure by observing changes in the color of the second coating
resulting from eruption of microcapsules in the first coating.
Inventors: |
Bilder; Wayne H. (Northampton,
PA), Granata; Richard D. (Bethlehem, PA), Leidheiser,
Jr.; Henry (Venice, FL) |
Assignee: |
Competitive Technologies Inc.
(Bethlehem, PA)
|
Family
ID: |
23445687 |
Appl.
No.: |
08/367,039 |
Filed: |
January 3, 1995 |
Current U.S.
Class: |
427/8;
73/104 |
Current CPC
Class: |
B05D
5/00 (20130101); B05D 5/06 (20130101); G01N
19/08 (20130101); G01N 21/91 (20130101); G01N
2203/0064 (20130101); G01N 2203/0652 (20130101) |
Current International
Class: |
B05D
5/06 (20060101); B05D 5/00 (20060101); G01N
19/08 (20060101); G01N 19/00 (20060101); G01N
21/91 (20060101); G01N 21/88 (20060101); G01N
3/00 (20060101); G01N 3/06 (20060101); B05D
003/00 () |
Field of
Search: |
;73/104,105 ;252/408.1
;427/8,146,650 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Cheu, Y. F., "Automatic Crack Detection with Computer Vision and
Pattern Recognition of Magenetic Particle Indications", American
Society For Nondestructive Testing, Inc., Materials Evaluation, 42,
Nov. 1984, pp. 1506-1510. .
"Eddy Current Imaging Device Offers Prospect of Simple Crack
Detection"Aviation Week & Space Technology, Jan. 22, 1990, p.
96. .
Hopwood, Theodore II, P. E., "Acoustic Emission Inspection of Steel
Bridges" Public Works, May 1988, pp. 66-69. .
Dunker, Kenneth F., Rabbat, Basile G., "Why America's Bridges Are
Crumbling"Scientific American, Mar. 1993, pp. 66-72. .
Kuvin, Brad, F., "Expert Inspection of Steel Bridges", Welding
Design and Fabrication, Aug. 1988. .
"Microencapsulation" Encyclopedia of Polymer Science And
Engineering, vol. 9, Second Edition, 1987, pp. 724-745. .
Pillai, S. A., et al., "Detection and Characterization of Tight
Cracks Using Photoelastic Coatings", The American Society For
Nondestructive Testing, Inc., Materials Evaluation, Mar. 1992, pp.
367-371. .
DeForest, A. V., Ellis, Greer, "Brittle Lacquers as an Aid To
Stress Analysis", Journal of Aeronautical Sciences, 1940, pp.
205-208..
|
Primary Examiner: Lusignan; Michael
Attorney, Agent or Firm: Novack; Michael R.
Government Interests
This invention was made with U.S. government support awarded by the
Office of Naval Research, Grant No. NOOO14-89-J-1089. The U.S.
government has certain rights in this invention.
Claims
We claim:
1. A method for monitoring a structure for the formation of cracks
and for providing protection of the structure from the environment,
comprising the steps of:
(a) applying to the structure a first coating comprising
microcapsules of a first color;
(b) applying over the first coating, a second coating having a
second color; and
(c) identifying cracked portions which form in the structure by
observing changes in the color of the second coating resulting from
eruption of microcapsules in the first coating.
2. The method of claim 1, wherein said coating comprises a white
nitrocellulose lacquer.
3. The method of claim 2, wherein said microcapsules comprise
gelatin walls enveloping an oil soluble dye.
4. The method of claim 3, wherein said gelatin walls have a
diameter of between 50 and 250 micrometers.
5. The method of claim 4, wherein the oil soluble dye is red.
6. The method of claim 5, wherein the weight of the microcapsules
is between 5% and 20% of the weight of the coating.
Description
BACKGROUND
A large portion of the highway infrastructure is in need of repair.
Studies indicate that nearly a quarter of the half million highway
bridges in the United States have been classified as structurally
deficient. Each year, on average, between 150 and 200 spans suffer
partial or complete collapse. Current estimates for repairing
deficient bridges are of the orders of $100 billion.
The potentially catastrophic consequences of fatigue cracking can
be avoided by the early detection of fatigue cracks. Additionally,
early detection of cracks can significantly reduce the cost of
repairs.
A national mandate requires that bridges be inspected at least
every two (2) years. The most frequently used method of inspecting
bridge components for fatigue damage has been the elementary method
of visual inspection. In order for the inspection to be of any
value, the inspectors must find flaws early and accurately judge
which ones need immediate repair, and which ones can wait. Although
an inspector will typically examine the entire structure, a high
percentage of cracks occur in common or "critical" locations. The
most revealing sign of a crack is the existence of rust, oxide film
and powder. However, since rust does not always appear immediately
after a crack is formed, cracks may go undetected during visual
inspection.
Once a crack is observed or suspected, the structure is further
tested to determine the extent or severity of the damage. A number
of techniques are currently used to confirm the existence of a
crack. One well-known technique is the application of a
dye-penetrant to the crack in question. A liquid penetrant is
applied evenly over the surface being tested and allowed to enter
open discontinuities. The excess surface penetrant is removed by
wiping and the surface is dried. A developer is then applied,
drawing the penetrant out of the discontinuity, staining the
developer.
Another widely used technique is magnetic particle testing. This
test method is used to detect cracks in steel by applying a
magnetic field through the surface with a permanent or
electromagnet. The specimen is then sprayed with an ink containing
fine magnetic dust. The difference in flux density at a crack
causes the particles to be attracted to the crack, which makes the
crack visible.
Another method, ultrasonic testing, involves the transmission of
ultrasonic pulses by piezoelectric transducers through a material.
Changes in the amplitude of the received signal indicates the
presence of a crack or flaw.
Still another technique, Eddy Current Imaging, measures changes in
electrical impedance, produced in a material by an induction coil.
A flaw changes the detectable current.
Acoustic emission testing involves the monitoring of transient
waves resulting from energy releases due to crack growth.
X-ray and penetrating radiation methods are also used for flaw
detection in materials.
Brittle Lacquer coatings have long been recognized as a means for
evidencing the existence of strain in a material. These coatings
crack in response to the substrate. In order to be of quantitative
value, the coating must be used in a controlled environment, and
applied in a precise and uniform manner. The lacquer coatings are
also limited to work that can be closely observed so that the
cracks in the lacquer can be seen. These brittle coatings are of
little use for corrosion protection.
Photoelastic coatings are another known crack detection technique.
Photoelastic materials, when subject to a stress or strain field,
exhibit bifringence, which is seen as a fringe pattern when viewed
through a polariscope.
The aforementioned prior art detection techniques require the
initial step of observation of a crack by a trained inspector.
Additionally, many of these techniques require complex machinery,
and/or cumbersome and expensive physical removal of the structural
member in question on order to confirm the existence of, or
evaluate the extent, of the crack.
Visual inspection of bridge components on site, and at susceptible
locations of the structure, remains the first line of action. Thus,
there is a current need to improve an inspector's ability to detect
cracks during an initial visual inspection. Early detection will
result in savings of time and money. Making inspections more
accurate will reduce the number of inspections needed, and will
minimize repair costs.
Among the objects of the present invention is to provide a method
for aiding in the early detection of cracks in a structure. Another
object of the present invention is to provide an improved
protective coating for structures. A still further object of the
invention is to provide a self-activating crack indication system
visible to observers with minimal training. A still further object
of the present invention is to provide a non-destructive crack
indication technique.
SUMMARY OF THE INVENTION
The present invention utilizes microencapsulation to advantage in a
crack detection system. Microencapsulation is the envelopment of
small solid particles, liquid droplets, or gas bubbles within a
coating. Microcapsules are characterized as having a size between 1
and 1000 microns, but can have a wide range of geometries and
structures. A microcapsule contains an internal phase, and an outer
coating material.
Microcapsule-based products are used in a variety of industries
including: pharmaceutical, graphic art, pesticide, and food
industries. For instance, in the pharmaceutical industry,
microencapsulation provides an effective mechanism for controlled
released drugs. The largest application for microcapsules is in the
production of carbonless copy paper.
Many processes for preparing microcapsules have been reported in
the literature, these techniques include: pan coating,
centrifugation, biliquid column, electrostatic encapsulation, vapor
deposition, solvent evaporation, and gelation.
Most steel bridges, whether new or reconstructed, are painted. The
paint layer applied to the steel structure serves as a protective
coating against agents that cause corrosion, which weakens the
structure.
Applicant provides a means for simultaneously implementing a
crack-detection means, along with the application of a protective
coating of paint to a structure. One embodiment of Applicant's
method of structural crack monitoring includes the steps of:
applying a coating of a first color to the surface of the
structure, said coating including microcapsules containing a second
color, said microcapsules being subject to breakage upon occurrence
of a crack in said structure; and identifying cracked portions of
the structure by observing regions of said surface having said
second color. As used herein "color" can be any hue, as well as
black, white or grey. In another embodiment of Applicant's
invention, the a coating of a first color comprising the
microcapsules are applied to the structure, and a second coating of
a second color is applied over the first coating.
A primary benefit of applicant's method is that the ability to
identify a crack is greatly enhanced. Applicant's indication means,
which is automatically released when a crack develops, is
observable to the unaided eye at a distance of up to thirty
feet.
Applicant provides an improved method for the early detection of
cracks in a structure by applying a coating formulated with
microcapsules, which rupture in response to cracking of the
substrate upon which the coating has been applied, indicating the
formation of a fracture in the structure. Since the crack can be
observed at an early stage, a significant savings in money, and
potentially savings of lives, will result. For instance, if a crack
is detected at an early stage, a hole can be drilled at the end of
the crack to impede the propagation of the crack. Strain gages can
then be mounted to the member indicating whether or not the hole is
adequately relieving stress. Absent early detection capability, the
crack can propagate to a condition that will require replacement of
an entire bridge component, such as a girder. Further features and
advantages of the invention will become more readily apparent from
the following detailed description, when taken in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a photograph of an epoxy coated steel panel which has
been subjected to stress causing a crack in the structure.
FIG. 2 is a photograph of a nitrocellulose-coated steel panel
having the same coating as described in FIG. 1, which has been
subjected to a freeze-thaw cycling.
DETAILED DESCRIPTION
FIG. 1 is a photograph of a steel panel which has been subjected to
stress causing a crack in the structure. The panel was coated on
the bottom half with nitrocellulose lacquer containing
microcapsules comprised of an oil-soluble red dye having gelatin
walls ranging from 100 to 150 micrometers in diameter. An oil
soluble dye is preferable when using an oil-based paint such as a
lacquer, since the capsule wall material and the contents should be
mutually insoluble, and since the dye and the paint should not
degrade the capsule wall from either the inside or outside. If a
water based paint is used, the capsule wall would be oil soluble,
and the dye water soluble. The Microcapsules were supplied by Thies
Technology Inc., St. Louis Mo. The cracked region is indicated by
the eruption of the microcapsules.
FIG. 2 is a steel panel having the same coating as described in
FIG. 1. The steel panel in FIG. 2 has been subjected to a
freeze-thaw cycling. No stress forces were applied to the steel
panel in FIG. 2. As indicated by the photographs, the crack caused
by the stress fracture in FIG. 1, is distinctly different from the
cracks resulting from the freeze-thaw cycle. It is also expected
that the freeze-thaw cracks can be eliminated by modification of
the paint properties.
Additional testing was performed demonstrating the early detection
capability of the present invention. The microcapsules used in the
experiments, purchased from Thies Technology, Inc, St. Louis Mo.,
contained an oil-soluble red dye having gelatin walls ranging from
100 to 150 micrometers in diameter. These microcapsules were then
mixed with a white nitrocellulose lacquer prior to application. The
weight of the dye capsules was 8 to 11% of the paint weight. This
coating was then applied to low-alloy steel beams and boxes.
Varying stress ranges were then applied to the beams and boxes. In
one series of test, I-beams were supported on their ends, and
hydraulic presses were used to apply force at strategic locations
along the length of the beams. Cracks in the structure, indicated
by the presence of released dye from the ruptured microcapsules,
were observable to the naked eye when the cracks reached three (3)
millimeters in length. Stress continued to be applied to the beam
until failure occurred. The cracks were observable as released dye
when between 19% and 40% of beam lifetime remained.
The invention has been described with reference to particular
embodiments, but variations within the spirit and the scope of the
invention will occur to those skilled in the art. For example, it
will be understood that suitable microcapsules can be mixed with
other types of paints, such as urethane/acrylic,
bisphenol-A/epichlorohydrin epoxy, or other paints typically used
on structural steel. Also, the wall materials of the microcapsules
can be a variety of materials, such as variants of gelatin (gum
arabic, polyphosphate, carbooxymethylcellulose, and carragenan),
urea formaldehyde, melamine formaldehyde, polyurea, or polyamide.
Additionally, it is understood that other colors of dye could be
used in the microcapsule. Also, while the invention is particularly
applicable to bridges, it can be employed on various other
structures, examples being buildings, vehicles or aircraft.
Further, a fiber optical light pipe or other optical transfer means
can be configured to monitor locations not directly visible to an
observer (e.g. visually obstructed areas, physically inaccessible
areas, etc.). Also, in some applications, equipment which
automatically detects color change could be employed.
* * * * *